Stay away from the dealer
All the dealers do is send it out to a cheap rust proofing center and have it done. Some actually do it in house,but either way they mark it up HUGE. Around here it is $4-500 at the dealer and $1-200 at rust check,etc. Also the big rust proofing co. stand behind their warranty,as stated above. The dealers usually say they won't fix it until it is "perforated" (ie:hole),and then they say it was "road damage" (ie:stone chip which caused the rust to start),which is not the fault of the rustproofing.